In the formation of a Wafer-Level Chip Scale Packages (WLCSP), integrated circuit devices such as transistors are first formed at the surface of a semiconductor substrate in a wafer. An interconnect structure is then formed over the integrated circuit devices. A metal pad is formed over, and is electrically coupled to, the interconnect structure. A passivation layer and a first polyimide layer are formed on the metal pad, with the metal pad exposed through the openings in the passivation layer and the first polyimide layer.
A Post-passivation interconnect (PPI) is then formed, followed by the formation of a second polyimide layer over the PPI. An Under-Bump Metallurgy (UBM) is formed extending into an opening in the second polyimide layer, wherein the UBM is electrically connected to the PPI. A solder ball is then placed over the UBM and reflowed.
A molding compound is then applied to protect the solder ball. In the application of the molding compound, a liquid molding compound is applied, followed by pressing a release film on the liquid molding compound to squeeze out excess liquid molding compound. As a result, the top portion of the solder ball is exposed through the liquid molding compound. The liquid molding compound is then cured. After the curing of the liquid molding compound into a solid state, the release film is removed. The wafer is then sawed into a plurality of dies.